Laminin, a major glycoprotein component of basement membrane (BM) has been shown to promote attachment and motility of tumor cells and play a role in the differentiation of a variety of cell types. The major source of laminin and reconstituted BM (Matrigel) is the mouse EHS tumor which produces large amounts of BM components. A previous finding of laminin-mediated increase in type IV collagenolytic activity prompted a study on the identification of metalloproteinases involved. A variety of normal and neoplastic cell lines were incubated with laminin, the E8 fragment of laminin, or the synthetic laminin peptides Y-I-G-S-R and PA22-2 which contains the putative cell binding sequence, I-K-V-A-V. No modulation in the mRNA expression of matrix metalloproteinases (MMP-1, MMP-2, MMP-3, MMP-9) or the tissue inhibitors of metalloproteinases (TIMP-1, TIMP-2) was detected in the presence of laminin, or the laminin derived fragments. Substrate gel electrophoresis of the laminin and E8 laminin fragment revealed several gelatinolytic metalloproteinase species. Quantitation of type IV collagen degradation by laminin containing culture supernatants and the laminin and E8 fragment alone demonstrated that most of the laminin-mediated increase in type IV collagenolytic activity could be accounted for by the presence of contaminating metalloproteinses in the laminin preparation. The presence of MMP-2 and MMP-9 in the laminin and the E8 fragment of laminin was verified by Western blotting, using monospecific peptide antibodies. Matrigel was found to contain identical MMP profile. These findings, together with other reports on the presence of proteinases and growth factors in reconstituted BM, should be taken into account in the interpretation of the effects of Matrigel and laminin on cell behavior.